Our long term aim is to find the mechanisms by which biosynthesis of cholesterol is regulated in mammalian cells, especially in liver. However, our initial experiences with rat liver, and human leucocytes have convinced us that a simpler model is required initially, and we have chosen to study regulation of ergosterol biosynthesis is yeast by a combination of genetic and enzymic techniques which we have used in aromatic biosynthesis in E. coli, and other workers have used in many pathways in bacteria, yeast and Neurospora. It is hoped that knowledge of sterol regulation in yeast will help in approaching similar metabolic problems in mammalian cells. Anaerobically growing yeast requires addition of a sterol, owing to participation of oxygen in several reactions of ergosterol biosynthesis. Anaerobic growth for several generations without sterol results in cells without internal structure, i.e., cytoplasmic membrane, nuclear membrane, vacuoles and mitochondria. We have obtained yeast mutants which cannot synthesize ergosterol or which overproduce ergosterol and transport it into the medium. We shall continue our studies of these mutants in order to determine the altered genes, enzymes and membranes responsible for their properties. Work is also in progress to isolate additional structural and regulatory sterol mutants. Our previous studies on regulation and enzymes of aromatic amino acid biosynthesis will be continued in part, but will be phased out.